1. Field of the Invention
The present invention relates to metal contacts to silicon substrates and other semiconductors in which the contact material includes a supply of dopant atoms, thereby acting as its own dopant source, to facilitate the formation of a low-resistance ohmic contact between the contact material and the substrate.
2. Description of the Background Art
In a properly designed p-n junction solar cell, the electrons move to the metal electrode which contacts the n-type silicon, and the holes move to the metal electrode which contacts the p-type silicon. These contacts are vitally important to the performance of the cell, since forcing current across a high resistance silicon/metal interface or through a high resistance electrode material robs useful power from the cell. The total specific series resistance of the cell, including interfaces and electrode material, should be no more than 1 xcexa9-cm2.
The need for a low-resistance contact places a fairly demanding requirement on the concentration of dopant atoms at the surface of the semiconductor. For n-type silicon, this dopant concentration must be xe2x89xa71xc3x971019 atoms/cm3 (which is 200 parts per million atomic (ppma) based upon a density for silicon of 5xc3x971022 atoms/cm3). For p-type silicon the requirement is less severe, with a surface concentration xe2x89xa71xc3x971017 atoms/cm3 (2 ppma) being required. Furthermore, to maximize the light energy to electrical energy conversion efficiency it is often desirable to have a lower surface doping concentration everywhere on the illuminated side except directly beneath the metal electrode, especially for the n-type surface. Thus, an ideal contact material is one which supplies a liberal amount of dopant to the silicon immediately beneath it (also known as self-doping), has a high electrical conductivity, makes a mechanically strong bond to the silicon, and does not degrade the electrical quality of the silicon by introducing sites where electrons and holes can be lost by recombination. Finally, this ideal contact material should be inexpensive and should lend itself to being applied by an economical process such as screen printing.
A known contact material which possesses, to a significant extent, the above-described desirable properties, is aluminum. Aluminum possesses these properties when used for contacting p-type silicon and therefore forming the positive electrode in a silicon solar cell. This is due to the fact that aluminum itself is a p-type dopant in silicon. Aluminum can dope silicon, as part of a process which alloys the aluminum with the silicon, provided the processing temperature exceeds the aluminum-silicon eutectic temperature of 577xc2x0 C.
For conventional solar cell structures the lack of a material, comparable to aluminum, for contacting n-type silicon in order to form the negative electrode of a solar cell, makes the fabrication of a simple, cost-effective solar cell difficult. In a conventional solar cell structure with a p-type base, the negative electrode (which contacts the n-type emitter) is typically on the front (illuminated) side of the cell and the positive electrode is on the back side. In order to improve the energy conversion efficiency of such a cell, it is desirable to have heavy doping beneath the metal contact to the n-type silicon and light doping between these contacts. Thus, the conventional silicon solar cell structure presently suffers from a loss of performance because of the opposing demands for high doping density beneath the contact metal and low doping density between the contact metal areas.
Existing technology for solar cell contacts to silicon (Si) utilize a silver (Ag) paste with glass frit (e.g., Ferro 3347, manufactured by the Electronic Materials Division of Ferro Corporation, Santa Barbara, Calif.) fired at xe2x89xa7760xc2x0 C. The glass frit promotes adhesion of the Ag layer to the Si surface. Such a contact requires a Si substrate which already has a heavily-doped surface layer (sheet resistance less than 45xcexa9/xe2x96xa1). The interface between the Si and the contact material usually dominates the series resistance of the entire cell. Thus, this technology also forces the cell designer to create a surface layer which is more heavily-doped than desired in order to bring the interface resistance to an acceptable level.
Therefore, what is needed is a method and apparatus for self doping contacts to a semiconductor, said contacts being heavily doped beneath the bonding point to the semiconductor but lightly doped between the contacts, having high electrical conductivity, and a strong mechanical bond which is easily fabricated and cost effective.
The present invention provides a system and method for creating self-doping contacts to silicon devices in which the contact metal is coated with a layer of dopant, alloyed with silicon and subjected to high temperature, thereby simultaneously doping the silicon substrate and forming a low-resistance ohmic contact to it.
A self-doping negative contact may be formed from unalloyed Ag which may be applied to the silicon substrate by either sputtering, screen printing a paste or evaporation. The Ag is coated with a layer of dopant. Once applied, the Ag, substrate and dopant are heated to a temperature above the Agxe2x80x94Si eutectic temperature (but below the melting point of Si). The Ag liquefies more than a eutectic proportion of the silicon substrate. The temperature is then decreased towards the eutectic temperature. As the temperature is decreased, the molten silicon reforms through liquid-phase epitaxy and while so doing dopant atoms are incorporated into the re-grown lattice.
Once the temperature drops below the silver-silicon eutectic temperature the silicon which has not already been reincorporated into the substrate through epitaxial re-growth forms a solid-phase alloy with the silver. This alloy of silver and silicon is the final contact material, and is composed of eutectic proportions of silicon and silver. Under eutectic proportions there is significantly more silver than silicon in the final contact material, thereby insuring good electrical conductivity of the final contact material.
One possible advantage of the self-doping contact includes the elimination of the need for a pre-existing heavily-doped layer, thereby reducing the number of processing steps. The elimination of the heavily-doped layer also permits the use of a more lightly-doped emitter than is possible for existing technology. This increases cell efficiency because of the resulting higher cell photocurrent. Furthermore, adhesion of the contact to the Si surface may be improved over existing technology by specifying that alloying occur between Ag and Si. An alloyed contact is more adherent than a deposited contact, even if the deposited contact has glass frit. In addition, it has been demonstrated that an alloyed 146A contact remains intact after dipping in HF, unlike a deposited contact with glass frit which is dislodged from the Si substrate by immersion in HF. Such insensitivity to HF for alloyed contacts opens processing options not available with deposited contacts.
Other possible advantages of the invention will be set forth, in part, in the description that follows and, in part, will be understood by those skilled in the art from the description or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims and equivalents.